GEOPHYSICAL LABORATORY. 137 



(430) The granites of Washington, D. C. Henry S. Washington. J. Wash. Acad. Sci., 



11, 459-470. 1921. 



The granites of the District of Columbia occur in two long belts that extend 

 northwest from near Georgetown. They are intrusive into Archean gneisses. 

 There are two kinds of granite. The most abundant is a biotite granite, which 

 is well exposed in quarries along Connecticut Avenue and along Broad Branch. 

 The spur on which the Geophysical Laboratory stands is composed of this 

 granite. The silica percentage of the biotite granite varies from 67.5 to 69.0. 

 It contains a little oligoclase, scarcely any muscovite, and considerable 

 secondary epidote, but neither hornblende nor allanite. The second kind is a 

 muscovite-biotite granite, which is like the other, but contains about as much 

 muscovite as biotite and has about 72 per cent of silica. Both kinds of granite 

 are more or less foliated and their component minerals are much crushed, 

 effects due to pressure. 



Five new chemical analyses show their characters and indicate that the 

 granites are closely related to the less abundant diorites of the District. There 

 seems to be some evidence of magmatic differentiation in the intruded igneous 

 mass, in the sense that it is less silicic and more femic toward the borders. The 

 igneous rocks of the District have been little studied, but a complete study of 

 the geology and petrology of the District is highly desirable. 



(431) The distribution of population: A constructive problem. M. Aurousseau. Geogr. 



Rev., 11, 563-592. 1921. 



The rapid rate of increase of the world's population, especially of the white 

 races, is a matter of concern for geography and demands considerable exten- 

 sion of the present methods of study. The nature of the geographical problem 

 is here outlined, and a survey method of assessing the capacity of a given 

 region, the capacity being represented quantitatively as the expansion ratio 

 of the region, is evolved. It is maintained that a method of this kind is the 

 only means of obtaining an exact knowledge of the measure of the human 

 occupation of the habitable lands of the globe and of their resources. 



As a preluninary to the determination of the expansion ratio, the existing 

 data which are relevant to the matter are reviewed, with an attempt to system- 

 atize them. Urban groups, semiurban groups, and rural groups are con- 

 sidered in some detail, and the broader principles underlying their evolution 

 and distribution have been studied. The region of occupation is next con- 

 sidered, and among the numerous "natural regions" of current geography 

 four classes of regions of different status are distinguished. Cartographic 

 methods are discussed, and examples of the mapping of natural districts and 

 of power mapping are given. It is concluded that the expansion ratios of 

 regions of prolonged occupation, where the grouping of population has adjusted 

 itself to geographical conditions, may be determined on the basis of the existing 

 data, and that, if the subject be developed, a survey method of determination 

 is practicable for any region. 



(432) Obsidian from Copan and Chichen Itza. Henry S. Washington. J. Wash. Acad. 



Sci., 11, 481-487. 1921. 



Some Maya cores of obsidian brought back from Copan by Dr. Morley 

 were studied. They are of typical black obsidian, and much hke obsidians 

 along the zone of comagmatic igneous rocks that extends along the Rocky 

 Mountains and down the Andes. 



A bead of obsidian was examined which came from a cenote at the Maya city 

 of Chichen Itza, one of many objects thrown into it as sacrificial offerings. 

 This collection is now in the Peabody Museum and the numerous jades are 

 now under investigation. Chemical analysis of the obsidian bead shows that 

 it is of a very peculiar composition, being high in silica and soda but very low 



